U.S. patent number 4,349,408 [Application Number 06/248,038] was granted by the patent office on 1982-09-14 for method of depositing a refractory metal on a semiconductor substrate.
This patent grant is currently assigned to RCA Corporation. Invention is credited to Walter A. Hicinbothem, Jr., Ming L. Tarng.
United States Patent |
4,349,408 |
Tarng , et al. |
September 14, 1982 |
Method of depositing a refractory metal on a semiconductor
substrate
Abstract
A method for depositing a refractory on a semiconductor
substrate passivated with silicon dioxide and/or oxygen doped
polycrystalline silicon is disclosed. The usual undercutting of the
oxygen doped polycrystalline silicon or of the silicon substrate at
the edge where it meets the oxide is prevented by depositing a
layer of phosphorus doped polycrystalline silicon over the
passivation material before the metal is deposited.
Inventors: |
Tarng; Ming L. (Mercerville,
NJ), Hicinbothem, Jr.; Walter A. (Levittown, PA) |
Assignee: |
RCA Corporation (New York,
NY)
|
Family
ID: |
22937396 |
Appl.
No.: |
06/248,038 |
Filed: |
March 26, 1981 |
Current U.S.
Class: |
438/582; 257/636;
257/E21.163; 257/E21.309; 257/E21.578; 427/253; 427/307;
438/909 |
Current CPC
Class: |
H01L
21/28537 (20130101); H01L 21/76804 (20130101); H01L
21/32134 (20130101); Y10S 438/909 (20130101) |
Current International
Class: |
H01L
21/02 (20060101); H01L 21/768 (20060101); H01L
21/70 (20060101); H01L 21/285 (20060101); H01L
21/3213 (20060101); H01L 021/285 (); H01L
021/308 (); H01L 021/314 (); H01L 021/32 () |
Field of
Search: |
;156/628,653,656,657,662
;204/192E ;29/590,591 ;357/54,59 ;427/91,253,307 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
55-95339 |
|
Jul 1980 |
|
JP |
|
711167 |
|
Jan 1980 |
|
SU |
|
Other References
Morosanu et al. "Thin Film . . . Reactor" Vacuum, vol. 31, No. 7,
(1-19-81), pp. 309-313..
|
Primary Examiner: Massie; Jerome W.
Attorney, Agent or Firm: Morris; Birgit E. Cohen; Donald S.
Seitter; Robert P.
Claims
We claim:
1. A method of depositing a refractory metal on a silicon
substrate, said method comprising:
depositing an oxygen doped polycrystalline silicon layer on one
surface of said substrate;
depositing a phosphorus doped polycrystalline silicon layer on the
outer surface of said oxygen doped polycrystalline silicon layer,
said phosphorous doped polycrystalline silicon layer having a
phosphorous concentration of at least about 10.sup.19
atoms/cm.sup.3 ;
etching windows in said layers to expose said surface of said
substrate in the areas where said refractory metal is to be
deposited; and,
heating said substrate to a temperature in a range of between about
500.degree. C. to about 800.degree. C. and then exposing said
substrate to a gaseous refractory metal hexafluoride wherein the
refractory metal of said hexafluoride is selected from the group
consisting of tungsten and molybdenum.
2. A method in accordance with claim 1 wherein a layer of oxide is
grown on the outer surface of said phosphorus doped polycrystalline
silicon layer.
3. A method in accordance with claim 1 wherein a corrosion
resistant film is deposited on the outer surface of said refractory
metal.
4. A method in accordance with claim 1 wherein said oxygen doped
polycrystalline layer contains oxygen in the range of 2 to 45
atomic percent.
5. A method in accordance with claim 1 wherein the oxygen doped
polycrystalline silicon layer and the phosphorus doped
polycrystalline silicon layer are annealed at the same time an
oxide layer is thermally grown on the outer surface of said
phosphorus doped polycrystalline silicon layer.
6. A method in accordance with claim 1 wherein the substrate is
heated to a temperature of at least about 700.degree. C. to
maintain the reaction between said hexafluoride and the silicon
until about 500 Angstroms to about 2,000 Angstroms of metal is
deposited, thereafter, lowering the temperature of said substrate
to a temperature in a range of between about 500.degree. C. to
about 650.degree. C. and exposing said substrate to a mixture of
hexafluoride and hydrogen whereby additional metal is deposited on
said substrate.
7. A method in accordance with claim 6 wherein the ratio of
hydrogen to hexafluoride is at least about 10:1.
8. A method in accordance with claim 1 wherein a phosphosilicate
glass layer is grown over said phosphorus doped polycrystalline
silicon layer before said refractory metal is deposited, said
refractory metal being deposited by a chemical vapor deposition
technique whereby the refractory metal does not form on said
phosphosilicate glass and wherein a corrosion resistant metal layer
is deposited over said refractory metal and an adjacent portion of
said phosphosilicate glass layer.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of depositing a refractory metal
on a semiconductor substrate and, more particularly, to such a
method wherein a passivation material is also deposited around the
periphery of the metal.
In the semiconductor industry it is often necessary to deposit a
refractory metal on a predetermined surface area of a semiconductor
substrate and to also deposit a passivation material around the
periphery of that metal. As an example, Schottky diodes include a
metal contact on the surface of a silicon substrate to form a
rectifying junction. The passivation material surrounds the edge of
this contact to enhance the forward and reverse operating
characteristics of the diode. When manufacturing these diodes, it
is usual to coat the surface of the substrate with a passivation
material, form a window in the passivation material to expose a
predetermined surface area of the substrate and deposit the metal
on this surface area.
Various passivation materials have been utilized including silicon
dioxide (SiO.sub.2), semi-insulating materials such as oxygen doped
polycrystalline silicon (SIPOS) and combinations thereof. The use
of oxygen doped polycrystalline silicon either alone or covered by
a layer of silicon dioxide has come to be preferred because of its
enhanced passivation properties. For providing the contact, the use
of refractory metals has been preferred because of their thermal
stability and, recently, the use of tungsten (W) and molybdenum
(Mo) deposited by Chemical Vapor Deposition (CVD) techniques have
become most preferred.
The use of a CVD technique provides significant advantages over the
more usual evaporation and sputtering deposition techniques. First,
the metal adheres better to the silicon substrate due to the
chemical interaction between the metal and the silicon. Second, a
desirable metal silicide is automatically formed beneath the
substrate surface adjacent the metal. This eliminates the need for
high temperature alloying of the deposited metal and also avoids
the exposure of the metal to the corrosive environment used in such
alloying. Third, the refractory metal is deposited on the silicon,
but not the usual outer oxide of the passivant system. This enables
the refractory to be completely covered by a corrosion resistant
film.
When forming schottky diodes using oxide or a combination of oxygen
doped polycrystalline silicon and oxide in combination with a
refractory metal contact deposited by CVD techniques, several
problems have been realized. A most critical problem involves the
formation of a severe undercut in the silicon substrate that is
formed at the edge of the window where the silicon meets the oxide
layer or where the oxygen doped polycrystalline silicon meets the
oxide layer, depending upon the particular passivation material.
This undercut is formed when the metal is deposited on the
substrate and it severely degrades the forward and reverse
operating characteristics of the device.
SUMMARY OF THE INVENTION
This invention provides a method of depositing a refractory metal
on a semiconductor substrate without undercutting the silicon
substrate during that process. This is accomplished by depositing
an oxygen doped polycrystalline silicon layer on one surface of the
semiconductor substrate and then depositing a phosphorus doped
polycrystalline silicon layer on the outermost surface of the first
layer. Thereafter, windows are etched in the layers to expose
surface areas of the substrate where the metal is to be deposited.
Afterwards, the refractory metal is deposited on the exposed
surface areas of the substrate.
Various other advantages resulting from this invention will be
pointed out where appropriate during the description of a detailed
embodiment.
BRIEF DESCRIPTION OF THE DRAWING
FIGS. 1-5 of the drawing illustrate, in section, a semiconductor
substrate at various times when practicing this invention to form a
Schottky diode.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The invention will be explained with particular reference to making
a Schottky diode, but it should be understood that this invention
can be used to make a variety of other devices. Referring now to
FIG. 1, there is illustrated a conventional wafer or substrate 10
of semiconductor material, preferably silicon, in which a plurality
of semiconductor devices are to be made. Specific reference will be
made to the manufacture of a single semiconductor device, but it
should be understood that a plurality of such devices are normally
formed on a single substrate. The substrate 10 has a pair of
parallel major surfaces 12 and 14 and is appropriately doped with N
type impurity atoms, for example, phosphorus, so as to have a
heavily doped region, denoted by N+ on the drawing, adjacent the
surface 14 and a more lightly doped region, denoted by N, adjacent
the surface 12. While the particular description here involves the
use of N type dopant, P type dopant could also be utilized. Still
referring to FIG. 1 of the drawing, a layer 16 of semi-insulating
material is deposited on the surface 12 of the substrate. In
practicing this invention, the use of polycrystalline silicon doped
with oxygen atoms within a range of 2 to 45 atomic percent of
oxygen is preferred with a range of about 15 to 30 atomic percent
being most preferred.
The deposition of the oxygen doped polycrystalline silicon layer 16
is accomplished with a chemical vapor deposition (CVD) technique at
either low or atmospheric pressure. In using such a technique, the
substrate 10 is placed in a CVD reactor (not shown in the drawing)
connected, via suitable valving, to sources of nitrous-oxide
(N.sub.2 O) and silane (SiH.sub.4). The substrate is heated in the
reactor to a temperature of between about 600.degree. C. and
750.degree. C., for example to approximately 670.degree. C., and
the nitrous oxide and silane are fed into the reactor in a ratio of
about 0.2 to 0.4 for a period of time sufficient to deposit an
oxygen doped polycrystalline silicon layer 16 of about 5,000
Angstroms thick.
In accordance with this invention, a phosphorus doped
polycrystalline silicon layer 18 is deposited on the exposed
surface of the oxygen doped polycrystalline layer 16 so as to
completely cover the latter. The polycrystalline silicon layer 18
has a phosphorus doping concentration of at least about 10.sup.19
atoms/cm.sup.3 and has an initial thickness of about 3,000
Angstroms. Conveniently, this deposition can be accomplished by a
CVD technique in the same reactor used to deposit the oxygen doped
polycrystalline layer 16. To accomplish this, the reactor is also
connected via suitable valving to a source of hydrogen phosphide
(PH.sub.3), more commonly referred to as phosphine. After
deposition of the layer 16, the flow of the nitrous-oxide and
silane is discontinued and the substrate 10 is cooled to a
temperature of between about 550.degree. C. and 650.degree. C.,
preferably to about 600.degree. C., and that temperature is
maintained while phosphine and silane are fed into the reactor in a
ratio of about 1 to 3,000. The phosphine is preferably mixed with
nitrogen (N.sub.2) and the phosphine content of the mixture is
about 500 parts per million. This continues until the layer 18 is
deposited on the layer 16.
Referring now to FIG. 2, windows are formed in the layers 16 and 18
to expose surface areas 12a and 12b of the substrate 10. As will be
made clear hereinafter, the surface areas 12a are those areas where
metal contact is to be made to form the Schottky barrier and
surface areas 12b define the areas where the individual
semiconductor devices are to be separated from each other. These
windows are formed in accordance with generally conventional
photolithographic techniques wherein a photoresist is placed over
the exposed surface of the phosphorus doped polycrystalline silicon
layer 18 and is then masked and treated to define unprotected areas
corresponding to the surface areas 12a and 12b and to protect the
remaining area of the layer 18. With the treated photoresist in
place, the layers 16 and 18 are etched in a solution of nitric
(HNO.sub.3) and hydrofluoric (HF) acid, preferably a solution of
70% by volume nitric and 40% by volume hydrofluoric in a ratio of
about 99.0 to about 0.5. This removes the phosphorus doped
polycrystalline silicon and the oxygen doped polycrystalline
silicon to expose the silicon substrate in the areas 12a and 12b.
Because the faster etch rate of the phosphorus doped
polycrystalline silicon relative to the oxygen doped
polycrystalline silicon, the peripheral edges of the oxygen doped
polycrystalline silicon surrounding the exposed silicon surface
areas 12a and 12b are formed with a desirable taper illustrated at
16a in FIG. 2 of the drawing. The tapers 16a provide better
adhesion for the material subsequently deposited on the exposed
silicon surface areas 12a and 12b.
The photoresist is now stripped from the outer surface of the
phosphorus doped polycrystalline silicon layer 18 and the wafer
appears as illustrated in FIG. 2 of the drawing. At this point,
both the oxygen doped and the phosphorus doped polycrystalline
silicon layers 16 and 18, respectively, are annealed.
It has been found preferable to deposit an oxide layer 22 over the
now exposed surfaces 12a and 12b of the substrate 10 as well as
over the phosphorus doped polycrystalline silicon layer 18 and seen
in FIG. 3. In accordance with one aspect of this invention, the
oxide layer 22 can be formed while annealing the oxygen and
phosphorus doped polycrystalline silicon layers 16 and 18,
respectively. This can also be done in a CVD reactor connected, via
suitable valving, to sources of steam and hydrogen chloride (HCl).
The substrate 10 illustrated in FIG. 2 is placed in the reactor and
heated to a temperature of about 900.degree. C. The steam and about
1% to 10% hydrogen chloride acid are fed into the reactor for a
period of about 30 minutes to thermally grow a 2,500 Angstrom thick
oxide layer 22. On the exposed surface areas 12a and 12b of the
substrate 10 and on the exposed surface areas of the oxygen doped
polycrystalline silicon 16, silicon dioxide (SiO.sub.2) is formed;
on the exposed surface areas of the phosphorus doped
polycrystalline silicon layer 18, the phosphorus reacts with the
steam and a phosphosilicate glass (an amorphous mixture of silicon
dioxide and phosphorus pentoxide (P.sub.2 O.sub.5)) is formed.
Formation of the phosphosilicate glass is highly desirable since it
is an effective ion getterer which adds to the long term stability
of the device being formed. Additionally, the phosphosilicate glass
enables the subsequent selective deposition of metal and prevents
surface leakage.
After the oxide layer 22 is grown, its exposed surface is coated
with a photoresist which is then treated in accordance with
generally conventional photolithographic techniques to define
unprotected windows adjacent the silicon surface areas 12a the
tapered surface areas 16a and a peripheral surface area 18a of the
phosphorus doped polycrystalline silicon layer. The peripheral
surface area 18a is located adjacent to and outwardly of the
tapered surface 16a. Once these windows have been formed, the
unprotected oxide layer 22 is etched away in a buffered
hydrofluoric (HF) acid to expose the silicon surface areas 12a, the
tapered edges 16a of the oxygen doped polycrystalline silicon layer
16, and the peripheral edges 18a of the phosphorus doped
polycrystalline layer 18 as illustrated in FIG. 4. The oxide layer
22 is not removed adjacent the silicon surface layers 12b so that
when the devices are separated from each other, the oxide is in
place for passivation purposes.
With the substrate 10 in the condition illustrated in FIG. 4, it is
now ready for metal deposition to form the Schottky diode. To
accomplish this, a metal contact 24 is deposited over the exposed
silicon surface areas 12a. Refractory metal is preferred for the
contact 24 and the use of tungsten or molybdenum deposited by CVD
techniques is most preferred. In accordance with a CVD technique,
tungsten (W) can be deposited on the surface areas 12a by placing
the substrate as illustrated in FIG. 4 in a CVD reactor and heating
the substrate to a temperature within the range of about
500.degree. C. to about 800.degree. C. Tungsten hexafluoride
(WF.sub.6) and an inert carrier gas such as argon (Ar) or nitrogen
(N.sub.2) are then fed into the reactor and the tungsten
hexafluoride will react with the silicon in accordance with the
following:
Preferably, the temperature of the substrate 10 is at least about
600.degree. C. to about 800.degree. C. and most preferably is about
700.degree. C. so that a tungsten silicide layer is formed in the
silicon beneath the exposed surface areas 12a. Afterwards, tungsten
atoms will replace silicon atoms in accordance with the above
reaction and a tungsten layer 24 forms in the exposed surface areas
12a, the tapered surfaces 16a, and the exposed surfaces 18a. The
tungsten will not form on the phosphosilicate glass portion of the
oxide layer 22, but trace amounts may be deposited on the silicon
dioxide portion.
The deposition of tungsten will stop after a layer of from about
500 angstroms to about 2,000 Angstroms thick has been deposited.
Inasmuch as a layer of about 4,000 Angstroms thick is desired, the
deposition process must be altered. In order to deposit the
additional 2,000 Angstroms to 3,500 angstroms of tungsten, the
temperature is lowered from the 600.degree. C. to 800.degree. C.
previously used to a temperature of about 500.degree. C. to about
650.degree. C., preferably to about 550.degree. C. At this point,
hydrogen (H.sub.2) is added to the tungsten hexafluoride and
carrier gas. The tungsten hexafluoride will react with the hydrogen
to deposit the desired additional tungsten in accordance with the
following:
The ratio of hydrogen to tungsten hexafluoride should be at least
about and preferably greater than 10 to 1.
EXAMPLE OF TUNGSTEN DEPOSITION
A tungsten layer was deposited by atmospheric CVD on a substrate
prepared in accordance with the preferred Detailed Description
relating to FIGS. 1-4. This substrate was placed in a CVD reactor
chamber and the chamber was heated to a temperature of
approximately 700.degree. C. The chamber was pumped down to a
vacuum and was then argon back filled to provide an argon
atmosphere at a positive pressure. Additional argon, functioning as
a carrier gas, was fed into the reactor chamber at the rate of 3
liters/minute and tungsten hexafluoride was pulsed into that
chamber at the rate of 15 cc/minute. A total of 14 pulses were fed
into the chamber with each pulse being 20 seconds in duration. The
elapsed time between pulses was 40 seconds. This process formed the
tungsten silicide layer and deposited a layer of tungsten on the
substrate, the layer having a thickness of about 1,000 Angstroms.
No tungsten was deposited on the oxide surface during this
deposition process.
When the reaction in accordance with the first equation above
stopped, the temperature in the reactor chamber was lowered to
approximately 550.degree. C. Hydrogen was then fed into the chamber
at the rate of 8 liters/minute and argon flow at the rate of 3
liters/minute was continued. Tungsten hexafluoride was again pulsed
into the chamber at the rate of 15 cc/minute. A total of ten pulses
were fed into the chamber with each pulse being 20 seconds in
duration. The elapsed time between pulses was again 40 seconds.
Additional tungsten was deposited on that previously deposited and
the total thickness of the tungsten layer was approximately 4,000
Angstroms. When the substrate was examined, neither the silicon nor
the oxygen doped polycrystalline silicon were undercut. Tungsten
islands were formed on the oxide surface during this second
deposition process. These isolated tungsten islands do not conduct
current and, thus, will not adversely affect the quality of the
passivation. Moreover, these islands improve the adhesion of any
subsequent metal deposited over the oxide surfaces.
Returning to the detailed description of a preferred embodiment, it
is desirable to deposit a corrosion resistant metal film over the
tungsten and any suitable metal and depositing technique can be
utilized. It is preferred that a platinum layer 26, as illustrated
in FIG. 5, be deposited with a thickness of about 2,000 Angstroms.
This can be accomplished by placing an additional photoresist over
the substrate, and then processing it in accordance with generally
conventional photolithographic techniques to expose a peripheral
portion 22a of the oxide layer 22 adjacent the tungsten layer 24 as
well as the entire surface area of that tungsten layer. The
platinum is then deposited in accordance with any conventional
technique and, the photoresist is removed. The platinum deposited
on the photoresist can be removed by the "lift-off" technique. This
is done by immersing the substrate 10 into a stripper such as
acetone. As the photoresist is dissolved, the platinum layer on it
is lifted off and removed automatically.
As seen in FIG. 5, the outer peripheral edge of the tungsten layer
24 is covered and protected by the platinum layer 26. This is in
contrast to conventional deposition methods wherein the outer edge
of the contact metal is exposed and subject to corrosion and
degradation during subsequent processing.
A plurality of Schottky diodes have now been formed on the
substrate 10. Further processing is in accordance with generally
accepted techniques to provide further metallization layers,
separate the individual diodes from each other at the surface areas
12b and package the individual diodes.
From the foregoing it can be seen that a method has been provided
for depositing a refractory metal on a semiconductor substrate
utilizing passivant materials for the mask without undercutting the
silicon substrate or the oxygen doped polycrystalline silicon. In
addition, various other advantages of the technique have been
pointed out in the detailed description of a preferred embodiment
of the invention. It should be obvious to those skilled in the art
that various changes and modifications can be made without
departing from the true spirit and scope of the invention as
recited in the appended claims.
* * * * *